DESCRIPTION (Applicant's Description): The applicant's career goal to combine research with clinical practice of hematopathology is a logical outcome of his c o n t inued interests in carcinogenesis, cellular differentiation, and transcription factors. While he has had a productive research experience with skeletal myocytes and B cell transcription factors, this proposal represents a significant change in research subject and uses many technical approaches that are novel to him. The expression of the hematopoietic-restricted transcription factor c-myb can be disrupted using antisense oligonucleotides and this leads to cell cycle arrest or apoptosis of leukemic cells. Based on these findings, clinical trials of antisense oligonucleotide based therapy against leukemic cells are in progress. Although c-myb is a rational target, it is still less than perfect for treatment of leukemias because it is also expressed by normal hematopoietic cells and is essential for their normal development. A better fundamental understanding of c-myb biology may identify additional and possibly better targets. MYB may play a role in leukemogenesis by physically interacting with transcription co-factors and activating specific genes that promote cellular proliferation. These co-factors and c-myb activated genes are potential targets for antisense based therapies against leukemias. It is possible that some of these targets are essential for cell cycle progression in some leukemic cells but dispensable in normal hematopoietic progenitors. To test this, the expression of known c-myb activated genes and interacting proteins will be suppressed using antisense approaches and cellular proliferation of normal and leukemic cells will be measured. Novel c-myb activated genes will be identified using differential display. The expression of these genes will be suppressed using an antisense approach and the effect on cellular proliferation will be measured. In some leukemias, c-myb may be mutated resulting in altered protein interaction or gene activation. Primary leukemias will be screened for spontaneous c-myb mutations and these mutants will be analyzed for their effects on cellular proliferation using cell counting or tritiated thymidine incorporation, gene activation using RT-PCR or northern blot analysis, and co-factor interaction using co-immunoprecipitation or mammalian two hybrid assay.